The present invention relates to a system for controlling a sound field for a multi-speaker system wherein an audio signal is reproduced for providing an effect as if the listener is in a concert hall.
The sound field control system renders it possible to provide a sound effect, in a home listening room, similar to that of a reflected sound environment such as a concert hall. More particularly, delayed audio signals corresponding to the reflected sound of the direct sound is added to the original audio signal representing an original direct sound, and the composite signal is reproduced by a multi-speaker system comprising four or more loudspeakers.
Referring to FIG. 6, in an example of the sound field control system, two main speakers a and b are disposed in front of a listener A for emitting the direct sound, and four speakers c to f are disposed in front and at the rear of the listener for emitting reflected sound. The listener A is thus able to sense a spacial impression of sound in regions B and C. More particularly, the reflected-sound speakers c and e reproduces delayed sound imitating an actually reflected sound which comes from a position g, as if in a hall. The delayed sound, in accordance with the principle of composition of vectors, is intended to cause the reflected sound to be localized at the position g. Similarly, the speakers d and f reproduces sounds reflecting from the opposite direction.
However, a time difference between sound waves reaching both ears (interaural) from the speakers c and e is actually so small, that the reflected sound can be heard not as being emitted at the position g, but at the respective positions of the speakers c and e. Hence the reproduced sound fails to provide a lateral expanse in sound, so that the listener cannot feel the sound surrounding him as in a hall.
Such an expansion feeling of sound field can be represented by a coefficient of correlation between both ears of the reverberation. The preferred interaural coefficient of correlation has a frequency response as shown in FIG. 7.
Since the right and left audio signals of a stereophonically recorded reverberation is non-correlated, the coefficient of correlation thereof is zero. It is known that when both the audio signals are imparted with a correlation coefficient corresponding to that of a sound at both ears in a diffuse sound field such as a reverberation room, a preferable spacial sound impression can be achieved with only two speakers. Japanese Patent Publication 60-840 discloses a system wherein the correlation coefficient of the recorded reverberation sound is controlled in accordance with the principle so that the reproduced sound gives a spacial impression.
However, since the correlation coefficient value of normalized cross correlation function at zero lag time in the actual hall changes at random during the reverberation, it is difficult to set an optimum interaural correlation coefficient.